The transmission and reception of audio information pervades our environment. Radio broadcasts and the audio channel of television broadcasts issue virtually unending streams of audio. Audio recordings are manufactured, distributed and played back on a variety of equipment. Entire industries exist for the creation and dissemination of such products.
The maintenance of records concerning the transmission and reception of such audio data is a daunting task which is faced in various forms by numerous entities. Broadcasters are required to keep logs of their transmissions. Both individual stations and their affiliated networks need to know, for example, that a particular sponsor""s promotion or xe2x80x9cspotxe2x80x9d, for which the station and network have received revenue, was in fact broadcast in accordance with the agreement with the sponsor. Similarly, the sponsor and/or its advertising agency require assurances that the sponsor has received full value for its payment, and that its spot was broadcast by the contracted-for station or stations, at the contracted time or times, in its entirety.
Producers and distributors of audio recordings can obtain valuable marketing data from the analysis of the time and frequencies of broadcast of their releases. Such data can be of particular value in the analysis of the efficacy of promotional efforts for a particular artist or recording in a particular market. Licensing organizations can use frequency-of-broadcast data in their royalty disbursement calculations.
Traditional methods for the collection and auditing of broadcast data has been both laborious, time-intensive, and costly. Typically, a monitoring organization would monitor the transmission of a station of interest and manually record the identity of a particular broadcast segment and its air time. The continuous monitoring of a station requires the continuous presence of a listener. If more than one station is to be monitored, additional monitoring personnel are required. While the recordation of broadcasts are sometimes utilized for subsequent review, current methodology still requires the playback and monitoring of the recording, and thus serves only to time-shift the monitoring process, without effecting its actual duration. It has been estimated that an overall accuracy of no more than 70% typically results from conventional monitoring methodologies.
Various methods have been proposed to automate the auditing process. Such proposals have included the monitoring of a broadcast for a particular audio xe2x80x9csignaturexe2x80x9d, which would correspond to a known signature for a given broadcast segment. Such methods have met with only limited success.
To be of value, an automated system for audio signal recognition and identification must be essentially inaudible to at least the typical listener. It also must be xe2x80x9crobustxe2x80x9d, providing a high degree of accuracy and being capable of detection after the audio track with which it is employed has passed through any of the variety of transmission and processing media and equipment typically utilized in conventional broadcasts. It also should be relatively immune from detection or modification.
The above and other objectives and purposes are met by the present invention, which masks a stream of digital data with a primary audio signal in a manner which does not perceptively change the quality of the primary audio signal. The combined composite signal is capable of recordation and transmission over a wide range of media, and demonstrates a high degree of covertness.
The digital format data stream to be inserted is preferably divided into a plurality of strings of a chosen length, and each of the strings is algebraically added or xe2x80x9cinserted,xe2x80x9d on a bit-by-bit basis, with the audio program material along a determined portion of the program material, the portion being chosen to maximize the covertness of the inserted data. In order to facilitate the subsequent identification and extraction of the inserted data, each insertion may comprise, in addition to chosen data, which may for example be specific to the program material, synchronization data of a consistent format which is searched for as part of the decoding process and which identifies the insertion of the digital data string.
The particular portion of the program material along which a data string is inserted is preferably determined by a process whereby the data string to be inserted is algebraically added with the audio program along each of a plurality of potential portions or sites within a given longer segment of the audio program. At each potential data bit insertion location along a particular portion of the program material, the amplitude required for each inserted bit is individually determined so that the audio program signal at that bit location will not cause an incorrect detection of that bit (i.e., a one detected as a zero or a zero detected as a one) when an attempt is made to extract the inserted data bit at the receiver. A series of tests is applied to each of the resulting composite signal portions, and the results of the tests are compared to determine which of the alternative portions produces a composite signal best demonstrating sufficient covertness. It has been found that one meaningful covertness criterion is the consideration of signal energy of the composite signal in particular frequency bands, over an interval of time equal to the insertion interval, with smaller energy deviations from that of the original signal indicating increasing covertness. Another meaningful covertness criterion has been found to be the extent of deviation of energy of the original signal from its mean value in a particular frequency band over the same time interval, with smaller deviations from the mean indicating increasing covertness.
Once the optimal location for the data string is found along a given segment or interval of the audio program and the insertion is performed, a next audio segment can be chosen for possible data insertion. Potential insertion portions are again identified and tested with the data string to be inserted therealong. The testing and insertion process can be repeated as desired through a series of potential insertion portions, and with data to be inserted, across any chosen total duration or extent of the audio program material.
The inserted digital data is retrieved from the composite signal by processing the composite signal, such as by filtering, to first eliminate portions of the signal which are unrelated to the inserted data. The remaining signal is compared to the known synchronization data pattern present in each insertion to identify the portion of the received signal which includes the inserted data string. The identification of the synchronization data pattern allows the remaining portion of the data string to be located and retrieved for subsequent processing. In a preferred embodiment, the composite audio signal is digitized, bandpass-filtered, matched-filtered, and threshold-detected. The detected bits are passed in a step-wise manner through a shift register which allows the composite signal to be compared on a step-wise, bit-by-bit basis with the sync data pattern. With use of a data signal of known length, the data bits can then be identified from their positional relationship to the identified sync data, and thus can be extracted and processed.